Four groups at the University of Minnesota that collaborate very actively since many years are studying a variety of metalloproteins and model complexes. Many of the proteins under study exhibit Electron Paramagnetic Resonance (EPR) signals characteristic of systems with integer electronic spin. The proteins contain either exchange-coupled iron-sulfur clusters of interacting chromophores as observed for the oxygen reducing site of cytochrome oxidase. Proteins under study include cytochrome C 554, hydroxylamine oxidoreductase, cytochrome oxidases, Fe 3 S4 and ZnFe 3 S4 clusters, methane monooxygenase and model complexes for iron-oxo proteins. In order to elucidate the nature of these novel EPR signals and in order to develop the theory of integer electronic spin systems further, we wish to study the proteins at S- and Q-band frequencies in addition to X-band. Presently, the four groups collaborate with over 30 groups at other U.S. institutions; a multifrequency capability should be of use for visiting collaborators.